In an accelerator using a high frequency electric field to accelerate charged particles, an accelerating tube is used as a device for generating the high-frequency accelerating electric field. Such an accelerating tube is preferable to accelerate the charged particles to a higher energy level while using less microwave power. It is said that the accelerating tube formed of superconductor material may serve the above purpose since the high-Frequency resistance on the tube wall thereof is small.
The conventional superconducting accelerating tube (FIG. 9) is constructed by working a hollow disk of superconducting material such as Nb into a half cell 1 in a dish form having a substantially constant thickness and having a small-diameter portion 2 and a large-diameter portion 3 which are open at the end portion thereof as shown in FIGS. 7 and 8 and then welding the half cells together into a tubular form. That is, the superconducting accelerating tube is constructed by arranging a plurality of half cells 1 with the small-diameter portion 2 and large-diameter portion 3 of each half cell set to face the small-diameter portion 2 and large-diameter portion 3 of adjacent half cells as shown in FIG. 9 and then respectively welding the small-diameter portion 2 and large-diameter portion 3 of each half cell to the small-diameter portion 2 and large-diameter portion 3 the adjacent half cells by use of an electron beam, for example, so as to connect the half cells.
With the above superconducting accelerating tube, it is impossible for a welding machine to approach a portion near the small-diameter portion 2 having the smallest diameter from the inside thereof since the diameter thereof is small. Therefore, when a plurality of half cells 1 are connected together by welding, it is required to weld the small-diameter portions 2 from the external surface side. However, since the wall thickness of the material of the half cell 1 is small, weld beads may easily occur on the internal surface side when the small-diameter portions 2 are welded together from the external surface side.
Since the electric field is strong near the small-diameter portion 2 of the superconducting accelerating tube, discharge may occur if the weld beads are left behind on the internal surface side. This is not preferable. Therefore, in the superconducting accelerating tube, smooth abrasion of the inner portion of the small-diameter portion 2, or the like, must be carried out after a plurality of half cells 1 are welded together.
Therefore, the half cell 1 is required to have a wall thickness (1 mm) larger than a certain value in order to make it possible to easily effect the welding operation, take a sufficiently large abrading margin, etc. after the welding operation, and have a sufficiently large strength which may prevent occurrence of deformation during the abrading process.
The characteristic of the superconducting accelerating tube largely depends on the heat conductivity thereof, and it is necessary to attain high heat conductivity and enhance thee cooling efficiency in order to store a large amount of energy.
That is, the superconductor has a high-frequency resistance so that a large amount of heat will be generated on the surface of the superconductor particularly in an electromagnetic resonator such as an accelerating Lube for storing a large amount of energy. Therefore, unless the heat is sufficiently quickly dissipated, the temperature of the superconductor rises and the superconductivity thereof will be destroyed before long.
Since the high-frequency excitation mode ordinarily used in the accelerating tube is TM.sub.010, the largest current will flow in a portion near the large-diameter portion 3 having the largest diameter and the smallest electric field. In contrast, in the small-diameter portion 2 having the smallest diameter, the electric field is high but the current is small. Since a large amount of heat may be generated in the large-diameter portion 3 in which a large current flows, it is necessary to enhance the cooling efficiency of the large-diameter portion 3.
As described above, in order to store a large amount of energy, it is necessary to enhance the heat conductivity of the superconducting accelerating tube and thus enhance the cooling efficiency. In order to attain this, it is preferable to enhance the cooling efficiency by reducing the wall thickness of the material of the superconducting accelerating tube.
However, in a case where the superconducting accelerating tube is constructed by welding as in the prior art, the possible degree of reduction of the wall thickness is limited.
As one of the prior art methods, there is used a method of enhancing the heat conductivity by enhancing the purity of the superconductor material such as Nb which constitutes the half cell 1 to increase the residual resistance ratio RRR. However, the method off increasing the RRR also has a limitation and it cannot be said that the present method is sufficiently good.
Further, as another prior art method, a half cell obtained by plating a superconductor material on a good heat conductor such as copper or aluminum has been developed. However, since the thickness of the superconductor material of the half cell is small, the plated superconductors cannot be welded together and therefore it is necessary to plate the superconductor on the joined portion after the half cells are joined.
This invention has been made in view of the above, and an object thereof is to provide a superconducting accelerating tube in which the wall thickness can be reduced to enhance the cooling efficiency and the half cells can be easily welded together.